Pulmonary arterial hypertension (PAH) is a disease of the small pulmonary arteries marked by a progressive increase in pulmonary vascular resistance, leading to right heart failure and ultimately death. PAH arises from different etiologies that share common pathophysiological features including remodeling of the pulmonary vessel wall, vasoconstriction, and thrombosis 1. This application focuses on the mechanism by which carbon monoxide (CO) exerts its therapeutic actions in a murine model of experimental pulmonary hypertension. Recent data reveals that CO is a critical effector molecule in ameliorating the progression and/or hastening the regression of vascular pathology in experimental pulmonary hypertension. Preliminary studies demonstrate linkage between the anti-proliferative actions of CO and its ability to prevent growth factor-induced downregulation of cav-1, a putative tumor suppressor gene. In addition cav-1 is an important regulator of endothelial nitric oxide synthase (eNOS) and is required for agonist and shear stress-stimulated NO production from eNOS. In the context of PAH, recent studies reveal downregulation of cav-1 and decreased eNOS activity in pulmonary hypertension. A central hypothesis provides focus to the proposed research plan - CO inhalation therapy exerts its beneficial effects on preclinical models of PAH by modulating the expression and interaction of caveolin-1 and endothelial nitric oxide synthase. This hypothesis will be tested by pursuing the following specific aims. Specific Aim I: Establish the efficacy of inhaled CO on vascular remodeling and endothelial cell dysfunction in preclinical PAH. Specific Aim II: Determine the role caveolin-1 in mediating the therapeutic effects of exogenous CO in experimental murine pulmonary hypertension. Specific Aim III: Determine the role of eNOS derived NO in mediating the therapeutic effects of exogenous CO in experimental murine pulmonary hypertension. The proposed research plan is based on a foundation of strong preliminary data and is designed to reveal a mechanistic role for cav-1 and eNOS/NO in the therapeutic effects of inhaled CO in PAH. The importance of the proposed research is underscored by the poorly understood mechanisms of CO-induced cytoprotection in pulmonary arterial hypertension. Upon completion of the proposed research we will have a better foundation upon which to investigate the novel therapeutic use of inhaled CO in this debilitating disease.